The MRI apparatus is a diagnostic imaging apparatus for medical use, utilizing nuclear magnetic resonance phenomenon due mainly to atomic nuclei of hydrogen. In general, a radio frequency magnetic field with a specific frequency is applied to a subject placed in a static magnetic field, simultaneously with a slice gradient magnetic field, thereby exciting nuclear magnetization within a cross section targeted for imaging. Next, a phase encoding gradient magnetic field and a readout gradient magnetic field are applied to provide the nuclear magnetization being excited with planar positional information, and a nuclear magnetic resonance signal generated from the nuclear magnetization is measured. The measurement of the nuclear magnetic resonance signal is repeated until filling measurement space referred to as k-space with the signals. An image is created from the signals being filled in the k-space by an inverse Fourier transform. Three types of gradient coils respectively associated with three axis directions orthogonal to one another apply the respective gradient magnetic fields. The RF magnetic field is applied by transmitting an RF pulse to the transmit radio frequency (hereinafter, referred to as “RF”) coil, and irradiating electromagnetic waves. In addition, a receive RF coil is used to measure the nuclear magnetic resonance signal.
The RF pulse and each of the gradient magnetic fields are applied based on a pulse sequence being predetermined. Various pulse sequences are known for any purpose. By way of example, in the pulse sequence of a gradient echo (GrE) type, the phase encoding gradient magnetic field is made to vary sequentially for each repetition time of the pulse sequence (TR), thereby measuring the nuclear magnetic resonance signals, the number of which is required for obtaining one tomographic image.
It is possible to set the RF pulse strength arbitrarily, according to a flip angle which is an imaging parameter. Here, the flip angle of 90 degrees indicates the RF pulse strength which maximizes a nuclear magnetic resonance signal (free induction decay signal: FID signal) to be measured, and the RF pulse with the flip angle of 90 degrees is referred to as 90° pulse. The RF pulse corresponding to the flip angle of 180 degrees is referred to as 180° pulse. The strength of each gradient magnetic field being applied is calculated and configured, on the basis of imaging parameters, such as a field of view of imaging, a reception band, and a size of measurement matrix.
An RF pulse being added before the pulse sequence, aiming at modifying image contrast, is referred to as a prepulse. A typical prepulse is an inversion recovery pulse which transmits the 180° pulse prior to any pulse sequence. A time duration from transmitting the inversion recovery pulse until measuring the nuclear magnetic resonance signal at the center of the k-space is referred to as “inversion time TI”, and the inversion time TI is adjusted to acquire the image contrast for any purpose.
Recently, in order to enhance the SN ratio of the image, the magnetic field in an MRI apparatus is developed to be magnetized higher, and an apparatus provided with the static magnetic field strength 3T or higher is coming into widespread use. A high magnetic field system allows acquisition of a high contrast image. On the other hand, there may occur a problem specific to this kind of high magnetic field system, that is, unevenness may occur in an abdominal image, or the like. Inhomogeneity in a rotating magnetic field which the transmit RF coil forms in an imaging region may be one of the causes of such image non-uniformity. This is called inhomogeneity in a transmitting sensitivity distribution (B1 distribution). This inhomogeneity occurs due to the reason as the following; when a magnetic resonance frequency of an electromagnetic wave to be irradiated becomes higher, along with the magnetization being higher, a wavelength of the electromagnetic wave within a living body becomes a scale almost equivalent to the size of the living body, and a phase of the electromagnetic wave is made to vary.
For reducing the inhomogeneity in the B1 distribution, there is suggested a method referred to as “RF shimming” which irradiates electromagnetic waves by using the transmit RF coil having multiple channels (e.g., see the Patent Document 1). This method controls phase and amplitude of the RF pulses provided to the respective channels, thereby reducing the inhomogeneity of the B1 distribution in the imaging region. Typically, in order to implement the RF shimming to achieve high homogeneity of the B1 distribution, the phase and amplitude provided to each of the channels are determined based on the B1 distribution created by each channel. Since the B1 distribution is dependent on the subject's body type, an organizational structure thereof, and the like, it is necessary to measure the B1 distribution for each channel as to each imaging portion of each subject.
As a typical method for measuring the B1 distribution, a double angle method is considered. This method calculates the B1 distribution by using images taken at an optional flip angle α and at its doubled flip angle 2α (e.g., see the Non Patent Document 1). In addition, there is suggested another method which acquires more than one image being different in the flip angle, and subjects the image signals being acquired to the fitting according to a theoretical formula as to image signal strength, the theoretical formula being defined for each pulse sequence, thereby calculating the B1 distribution (e.g., see the Non Patent Document 2). Alternatively, there is suggested a method for calculating the B1 distribution based on a cycle of signal strength variation without performing the fitting (e.g., see the Patent Document 2). Further alternatively, there is suggested another method which takes multiple images, while gradually varying the flip angle of a prepulse for the pulse sequence to which the prepulse is added, and calculates the B1 distribution based on the cycle of the image signal strength variation (e.g., see the Non Patent Document 3).